1. Field of the Invention
This invention relates to the repair of superalloy articles, and, more particularly, to an alloy powder mixture and a method for the repair of nickel-base and cobalt-base superalloy articles.
2. Description of the prior art.
Nickel-base and cobalt-base superalloys are used in high temperature operating gas turbine engine components because of the high temperature operating conditions. However, because such operating conditions are strenuous, cracks or other types of damage or deterioration can occur from such experiences as thermal cycling or airborne object impact, or their combinations. Also, discontinuities such as shrinkage, inclusions and cracks can occur during manufacture of such components. Because of the relatively high cost of such components, it is desirable to repair rather than to replace such articles.
One method for cleaning and repairing superalloy articles is described in U.S. Pat. No. 4,098,450--Keller et al issued July 4, 1978 and assigned to the assignee of the present invention. Described is a method using fluoride ions to contact oxide in a narrow crack or crevice to convert the oxide to gaseous fluoride. Subsequently, in one form of the invention, a repair alloy is applied to repair the crack or crevice.
In other repair methods which have been used for many years in the gas turbine engine art, oxides have been removed from parts to be repaired by mechanical means, or, predominantly in the case of cobalt-base alloys, through the use of hydrogen prior to application of a brazing alloy of the single alloy or wide-gap brazing mixture type. Such wide-gap brazing alloys have been described in the art, for example in U.S. Pat. No. 3,155,491--Hoppin et al issued Nov. 3, 1964.
Currently in use in certain gas turbine engines are nickel-base superalloy turbine vane castings one type of which is made from a nickel-base superalloy commercially available as Rene 77 alloy and consisting nominally, by weight, of 0.015% B, 0.07% C., 15% Cr, 4.2% Mo, 3.3% Ti, 4.3% Al, 15% Co, with the balance Ni and incidental impurities. It has been observed that such a vane casting can develop damage, such as areas of corrosion, airborne particle contact, a number of small cracks, or their combinations, as a result of operation at high temperatures. Currently, such cracks are repaired by methods such as welding or by use of the above-described method of the Keller et al patent. Repair alloys in the form of powder mixtures used in the Keller et al method heretofore include high and low melting components which are non-homogenous in that they are based on different alloy systems. In one example for the repair of Rene 77 alloy, the high melting component was Rene 77 alloy in powder form and the low melting component was a Ni-base-Cr-Co-Ta-Al-B alloy powder sometimes called D-15 alloy and described in U.S. Pat. No. 3,759,692--Zelahy, issued Sep. 18, 1973. Other powder mixtures include D-15 alloy with Rene 80 alloy and with X-40 alloy, both of which are commercially available. The result using such powder mixtures is a repair material which has hot corrosion resistance less than is desirable for present intended applications, particularly if the repaired article is not coated subsequently.